At that time they were emergency brakes, the shoes energize rolling forward because of their design (locks them tighter like every drum brake) reverse not to good. I’d advise taking drum off then look at the service manual picture then see if all the componites are on properly. If right the wheels-tires will drag ruts in dirt or gravel, because they were made to stop the truck when regular brakes failed, Ive had to do it a few times. BTW putting it in reverse will hold any truck I’ve ever had. If it’s rolling the brakes aren’t hold when you push the pedal either.
What I mean by energizing, as the brakes are applied they grab the drum which in turn tries to
rotate the shoes around, getting even tighter, works like a sprag in an auto trans.